Abstract:

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The subject of this work is to investigate viscoelastic properties such as loss modulus (G ́ ́), storage modulus (G ́), complex shear modulus (G*), complex viscosity (η*) and loss angle () at different temperatures by means of a small-amplitude oscillatory test. These properties allow to provide information about materials structure. For this purpose, we employed a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminum alloys and is the classic alloy for semisolid thixotropic studies. It is interesting to note that the Sn-15%Pb alloy exhibits a slightly decrease in storage modulus (G ́) over the entire frequency (0.01-10Hz) at high temperatures, showing its viscoelastic behavior. In addition, a detailed analysis of master curves (oscillatory tests) was made to relate the semisolid microstructure (solid fraction) with the plateau modulus (GN0) which is directly related with both molecular weight or percolation threshold in polymer and gels science respectively.

Abstract: Superplastic forming provides a good way for Ti alloys which are usually difficult to
be deformed. Ti75 alloy with a nominal composition of Ti-3Al-2Mo-2Zr is a newly developed corrosion resistant alloy, with a middle strength and high toughness. In the present paper, superplastic behavior of the alloy was investigated, the microstructural evolution in superplastic deformation was observed and the superplastic deformation mechanisms were analyzed. The results showed that the strain rate sensitivity, m, of the Ti75 alloy was larger than 0.3 and the strain was over 2.0 without surface cracking at 800°C and 5×10-4s-1 in compressive testing.
During the first stage of superplastic deformation, a phase grains became equiaxed, fine and homogeneous due to the recrystallization in a phase and diffusion in b phase. Newly formed equiaxed a grains then could slide and rotate, exhibiting superplastic features. The stress concentration caused by grain sliding of a grains could be released by slip and diffusion in b phase between the a phase grains, which acted as accommodation mechanisms.

Abstract: The demand for magnesium alloys has increased significantly during the past decade in the automotive and electronic industries where weight reduction becomes increasingly an important issue. At present, high-pressure die casting (HPDC) is a dominant process in production of magnesium alloy components. However, magnesium alloy components produced by HPDC suffer from porosity problem and this limits the enhancement of mechanical properties through subsequent heat treatments. The semi-solid processing (SSP) is an emerging new technology for near-net shape production of engineering components, in which the alloys are processed in the temperature range where the liquid and solid phases coexist. The SSP has various advantages over the conventional casting processes. It offers the castings with high integrity and less porosity and allows subsequent heat treatments for enhancement of mechanical properties. For these advantages, the SSP of
magnesium alloys has received increasing attention in recent years. In the present study, the continuous casting process was developed for the production of magnesium billets for the subsequent SSP. The process utilizes an electromagnetic stirring system in order to obtain desired microstructure with an excellent degree of homogeneity in both microstructure and composition. Prototypes of an air conditioner cover and a telescope housing were produced using the SSP of the continuously cast magnesium alloy billets.

Abstract: Warm compression tests of AZ31 Mg alloy were carried out at five temperatures in
30°C intervals from 210°C to 330°C. The samples of different thickness which were machined
from as-cast and pre-strained AZ31 billets were compressed into thickness 1mm and then cooled in
the air to room temperature. The microstructural evolution of AZ31 Mg alloy was investigated
during warm compression forming. The results show that all the samples have undergone a
microstructure changes to different scales in the range investigated. The twinning is the
predominant deformation mechanism for magnesium alloys at moderate temperatures and its
occurrence is dependent on temperature and strain. Microstructural evaluation indicates that the
mean size of the recrystallised grains decreases with increasing effective strain and temperature
because of sufficient dynamic recrystallization. The original grain has significant influence on
microstructural evolution during warm forming.

Abstract: The effects of alloy chemistry and heat treatment on the microstructure and mechanical properties of Ti-Nb-Zr-Mo-Sn near  type titanium alloys have been investigated. Near β titanium alloys consisting of non-toxic alloying elements Mo, Nb, Zr, Sn possess a low Young’s modulus, and moderate strength and plasticity. As the hot rolled TLM alloy (Ti-25Nb-3Zr-3Mo-2Sn) possesses high strength and low Young’s modulus a detailed investigation is performed for this alloy. Solution treatment of the hot rolled TLM alloy reduces strength and increases ductility without affecting the Young’s modulus. Ageing of the solution treated TLM alloy reduces elongation and increases the Young’s modulus with little change in strength. Both solution treated and aged conditions show features of two stage yielding associated with a strain induced martensitic transformation.

Abstract: The semi-solid slurry of A356 aluminum alloy was prepared through a serpentine pouring channel, which is a new method proposed recently for semi-solid forming process, and the effect of pouring temperature and bend number in the channel on the slurry microstructure was investigated and the slurry was finally rheo-die cast. The results show that when the pouring temperatures are between 640oC and 680oC, the slurry of A356 aluminum alloy with spherical primary a-Al grains can be prepared under the given conditions. The more the bend numbers in the channel are, the better the slurry is, i.e. the primary a-Al grains are more spherical and finer. The results also show that the as-cast ultimate strength and elongation of the rheo-die castings can reach 250MPa and 8.613.2% respectively. After T6 heat treatment, the ultimate strength and elongation of the rheo-die castings can reach 320MPa and 8.011.3% respectively. The work undertaken demonstrates eventually that the serpentine pouring channel process is a good method for semi-solid rheo-die casting or rheo-forming of metallic materials, the process is simple and the slurry cost is not expensive.